The present invention relates generally to multi-step dimming driver circuits for a light emitting diode (LED). More particularly, the present invention relates to providing a multi-step dimming LED driver which is insensitive to a particular load (e.g., an extremely light load).
A step dimming driver or ballast circuit as described in U.S. Pat. No. 7,843,141 is illustrated in FIG. 1. The step dimming driver of FIG. 1 has three power line inputs: two hot (black) and one neutral line. By sensing either how many or which hot line is connected to the driver, the driver can be controlled to dim a lighting output to a certain percentage of the full lighting output. For example, if only line #1 is connected (i.e., only switch S_1 is closed), the lighting output may be dimmed to 30%. If only line #2 is connected (i.e., only switch S_2 is closed), the lighting output may be dimmed to 70%. If both of line #1 and #2 are connected (i.e., both of switches S_1 switch S_2 are closed) the lighting output may be set to 100% output.
To achieve proper step dimming function, the line input or line connection must be reliably sensed at all kinds of load conditions (e.g., heavy load, light load, etc.). Conventional indirect line sensing methods such as those implemented by the circuit of FIG. 1 are very sensitive to the load condition.
As shown in FIG. 1, there are two voltage sensing branches: (1) resistors R3, R4 and R5 to sense the voltage across diode D6; and (2) resistors R6, R7 and R5 to sense the voltage across diode D8. The voltage at Vc (V_dim) is the superposed voltage from Va and Vb, as shown in FIGS. 2 and 3. When the connected load is large, the input rectifier will work continuously (or each diode in the rectifier bridge will conduct for a full half line cycle) so that the voltage across diodes D6 and D8 will form a half-wave sinusoidal shape. The voltage V_dim (Vc) may be either a continuous rectified full-wave (as illustrated in FIG. 2), or a half-rectified sinusoid waveform (as illustrated in FIG. 3). A dimming control signal used by the dimming control system of FIG. 1 may be determined by comparing an average voltage of Vc against a reference voltage.
When a connected load is very light (e.g., when an LED driver is operating in a lowest dimming mode), the line sensing method shown in FIG. 1 will not function properly. This is because when the load is very light, the diodes of the input rectifier bridge might not conduct full half-cycle.
What is needed, then, is an LED driver circuit that provides multi-step dimming that is relatively insensitive to load variations.